Comment spam is one of the most common forms of WordPress spam, if not the most common.

Here are some anecdotal statistics for this site. During the month of November 2014, snarfed.org received 796,990 POSTs to its comment form, wp-comments-post.php, vs just 2139 POSTs to xmlrpc.php (based on server logs). Of those, 9 comments and 3 trackbacks were ham. I also use XML-RPC to create and edit posts. So there was at least ~400x more comment form spam than pingback/trackback spam during that month.

Also posted on indiewebcamp.com/spam.